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On some stepsides it goes on the outside of the bed. That means the fender has to have a recess for the tire. I scrounged a fender like that off an abandoned stepside box. The spare tire mount was already gone, but I did measure the mounting hole locations. (assuming that really was a Ford box...)
If I remember correctly, on my '59 Merc pickup the spare tire location is under the bed. I don't know exactly HOW it mounts there; I think maybe the tire is supported by a bar, one end held by a bolt-like thing, and the other end screws into the frame somehow. I've never seen a complete one in real life or pictures; I'm guessing it wasn't a very convenient arrangement.
There's a how-to article here on adapting a minivan spare tire holder (like a little winch) to pickups.